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Unlock This CourseAll living things need energy to survive, grow and reproduce. Whether it's a tiny bacterium, a towering oak tree, or a sprinting cheetah, every organism must break down food molecules to release the energy stored within them. This process is called respiration and whilst the basic chemistry is similar across all life forms, different organisms have evolved fascinating ways to make it work efficiently.
Think of respiration as the body's power station - it takes in fuel (glucose) and oxygen, then produces energy (ATP) that cells can use immediately. But not all organisms do this in exactly the same way!
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Plants are brilliant at respiration! They carry out aerobic respiration in their mitochondria just like animals do. During the day, they also photosynthesise, which can mask their respiration. But at night, when there's no light for photosynthesis, you can clearly observe plants taking in oxygen and releasing carbon dioxide. Root cells are particularly active in respiration as they need lots of energy to absorb water and minerals from soil.
Most organisms prefer aerobic respiration when oxygen is available because it's incredibly efficient - producing 38 ATP molecules from just one glucose molecule. Let's explore how different groups of organisms have adapted to maximise their aerobic respiration.
Mammals and birds are respiratory superstars! They have highly developed lungs with millions of tiny air sacs called alveoli. These create an enormous surface area for gas exchange - your lungs have about 300 million alveoli with a total surface area roughly the size of a tennis court!
Alveoli are surrounded by capillaries, allowing rapid diffusion of oxygen into blood and carbon dioxide out. The walls are just one cell thick to speed up gas exchange.
A four-chambered heart pumps oxygenated blood efficiently around the body. This ensures all cells receive the oxygen they need for respiration.
Red blood cells contain haemoglobin, which binds to oxygen in lungs and releases it to respiring cells throughout the body.
Hummingbirds have the fastest metabolic rate of any bird! Their hearts beat up to 1,260 times per minute and they breathe 250 times per minute during flight. They need this incredible respiratory and circulatory efficiency to power their rapid wing beats - up to 80 beats per second. Without such efficient aerobic respiration, they couldn't hover or fly backwards like they do.
Life underwater presents unique challenges for respiration. Water contains much less dissolved oxygen than air, so aquatic organisms have evolved special adaptations to extract what they need.
Fish use gills instead of lungs. Gills are feathery structures with a massive surface area covered in tiny blood vessels. As water flows over the gills, oxygen dissolves into the blood whilst carbon dioxide diffuses out. The clever bit is that blood and water flow in opposite directions - this counter-current flow maximises oxygen extraction.
Counter-current flow means that even as blood becomes more oxygenated, it's still meeting water with higher oxygen concentration. This allows fish to extract up to 85% of available oxygen from water - much more efficient than our lungs!
When oxygen runs out, life doesn't stop - it adapts! Many organisms can switch to anaerobic respiration, though it produces much less energy (only 2 ATP molecules per glucose).
When you sprint or lift heavy weights, your muscle cells need energy faster than your blood can supply oxygen. They switch to anaerobic respiration, breaking down glucose to produce lactic acid. This is why your muscles feel sore after intense exercise - lactic acid builds up faster than it can be removed.
Muscles switch to anaerobic respiration when oxygen demand exceeds supply. Energy is still produced, but lactic acid accumulates.
After exercise, you keep breathing heavily to 'pay back' the oxygen debt and break down accumulated lactic acid.
Anaerobic respiration provides rapid energy for short bursts of intense activity, like sprinting or jumping.
Bakers and brewers have used yeast fermentation for thousands of years! When yeast cells run out of oxygen, they switch to anaerobic respiration, converting sugar into ethanol and carbon dioxide. In bread making, the CO₂ makes dough rise, whilst the alcohol evaporates during baking. In brewing, the alcohol is the desired product. This shows how anaerobic respiration can be incredibly useful to humans!
Microorganisms are the most diverse respirers on Earth. Some are obligate aerobes (must have oxygen), others are obligate anaerobes (oxygen is actually toxic to them!) and many are facultative - they can switch between aerobic and anaerobic depending on conditions.
Bacteria don't have mitochondria, so respiration happens directly in their cytoplasm and cell membrane. Some bacteria have evolved to use substances other than oxygen for respiration - they might use nitrates, sulfates, or even metals! This allows them to live in extreme environments where no other life could survive.
Some bacteria live in deep ocean vents, using sulfur compounds instead of oxygen for respiration. Others thrive in highly acidic or alkaline conditions that would kill most other organisms. Their flexible respiratory systems make this possible.
This often confuses students! Plants do both photosynthesis AND respiration. Photosynthesis makes glucose using light energy, whilst respiration breaks down glucose to release energy for the plant's activities. During daylight, both processes happen simultaneously, but respiration continues 24/7.
During the day, plants photosynthesise faster than they respire, so they're a net producer of oxygen. At night, only respiration occurs, so plants actually consume oxygen and produce carbon dioxide - just like animals do all the time!
When seeds germinate, they respire very rapidly to provide energy for growth. Before the first leaves develop, they can't photosynthesise, so they rely entirely on stored food and aerobic respiration. You can demonstrate this by placing germinating peas in a sealed container - they'll quickly use up the oxygen and produce measurable amounts of carbon dioxide and heat!
Evolution has produced amazing adaptations to make respiration more efficient. From the microscopic to the massive, organisms have found ingenious solutions to the challenge of gas exchange.
Insects use a network of tubes called tracheae that deliver oxygen directly to cells. No transport in blood needed - it's like having air conditioning ducts throughout their body!
Whales can hold their breath for hours by storing oxygen in their muscles and blood, slowing their heart rate and shutting down non-essential organs during deep dives.
Desert plants open their stomata at night to take in CO₂ for photosynthesis, minimising water loss whilst still allowing gas exchange for respiration.